1. Field of the Invention
The invention generally relates to laser-based material processing, and more specifically to laser based modification of microscopic target materials, for example link structures of a semiconductor memory device.
2. Description of the Related Art
One important application of laser based micromachining is severing conductive links on memory integrated circuits to improve memory yield by switching functional memory cells for defective memory cells. Several aspects of such laser based memory repair are disclosed in the LIA Handbook of Laser Materials Processing”, John Ready editor in chief, Laser Institute of America, Mongolia Publishing Co. Inc., 2001, Chapter 19, entitled “Link Cutting/Making” (hereinafter referred to as the “LIA-19”). LIA-19 discloses the use of conventional types of lasers for link blowing and memory repair, including diode pumped, solid state, q-switched lasers.
The assignee of the present application has developed a variety of laser systems for link processing, some of which are described in additional detail below. Reference is made to U.S. Patent Publication No. 2004/0188399 (hereinafter referred to as ‘Smart’), entitled “Energy-efficient, laser-based method and system for processing target material,” hereby incorporated by reference in its entirety, U.S. Patent Publication No. 2002/0167581 (hereinafter referred to as ‘Cordingly et al’), entitled “Methods and systems for thermal-based laser processing a multi-material device,” hereby incorporated by reference in its entirety, as well as U.S. Patent Publication No. 2004/0134896 (hereinafter referred to as ‘Gu et al’), entitled “Laser Based Method And System For Memory Link Processing With Picosecond Lasers,” also hereby incorporated by reference in its entirety.
Until recently, widely available commercial implementations of laser link severing machines have typically applied IR laser wavelengths in the 1.0 to 1.3 micron range to the links. In addition, pulse shaping, and pulse width selection have been beneficially applied to the links. As link width and pitch has become smaller over the years, implementation of shorter wavelength systems in the visible or UV ranges has begun to occur to take advantage of the smaller spot size that can be produced with shorter wavelengths. Laser processing with green light (e.g. about 500-550 nm) has been successfully performed commercially. UV laser processing machines have been developed, but high absorption at these wavelengths produces scattering, sensitivity to variations in dielectric layer thickness over the link, and other problems that have not been fully addressed to date.
Background information on shorter wavelength processing may be found in Muller et al, “Laser Process for Personalization and Repair of Multi-Chip Modules”, SPIE Vol. 1598, Lasers in Microelectronic Manufacturing, 1991. Muller reported using 0.3 mJ, 50 nanosecond (ns) pulses from a frequency doubled q-switched solid state laser (532 nm wavelength) to cleanly remove 15 micron gold lines from a MCM device (“double pulse cut”) without damaging an underlying polyimide layer.
In addition, U.S. Pat. No. 6,275,250 entitled “Fiber gain medium marking system pumped or seeded by a modulated laser diode source and method of energy control” (hereinafter referred to as the '250 patent) shows a fiber based MOPA system having a near IR semiconductor seed diode (1.1 micron wavelength) and multistage amplifier. FIG. 10 and the associated text of Cols 10-14 show the amplified output coupled to a frequency converter to produce a green laser output (550 nm) for marking, including marking of semiconductor (Silicon) substrates.
Research has also been ongoing regarding the effects of shorter pulses and groups of pulses for link processing. These developments have tended to increase the complexity, expense, and inefficiency of the laser source and related optical systems used to produce the laser energy incident on the links. Furthermore, various optical parameters of the laser beams are often difficult to control and are interdependent. Therefore, it can be difficult to optimize certain variables or to simultaneously optimize multiple laser beam variables and/or the performance of multiple optical components in the system.
Because laser systems have a wide variety of applications, research and development directed to addressing various undesirable aspects of laser performance has been ongoing for some time. For example, in Coldren et al, “Diode Lasers and Photonic Integrated Circuits”, John-Wiley & Sons, 1995, Chapter 8, laser diodes having controllable gain, phase, and wavelength were analyzed. It was suggested an output modulator stage be used (rather than an output phase modulator) so that a diode laser can operate CW and the emitted lightwave is modulated external to the cavity. The main reason for the modulator PIC (Photonic Integrated Circuit) is that external modulation adds less wavelength chirp in the process of modulation, and the modulation can be higher than that of a laser which is optimized for tunability or some other purpose. Additional laser system embodiments are illustrated in U.S. Pat. No. 6,868,100 and U.S. Application Publication 2007/0053391.
Still, application of laser technology to link processing requires improvement in efficiency and ease of use, and further development is needed in the art.